The present invention relates to a power supply circuit. More specifically, the present invention relates to a power supply circuit applicable to a control IC for improving the power factor of a self-excited power supply circuit.
A power factor control circuit is used in the power supply circuit, that switches on and off a full-wave-rectified AC input voltage to obtain a desired DC output voltage. The power factor control circuit includes an error amplifier, to which a feedback signal indicating the DC output voltage is inputted; a multiplier, to which the output of the error amplifier and an AC input voltage obtained by full-wave-rectification of the commercially supplied electric power are inputted; a sensing current comparator, that compares the output of the multiplier and an AC line current; a transformer, that receives the AC input voltage; a rectifying and smoothing circuit, that generates the DC output voltage; a switching device between the transformer and the rectifying and smoothing circuit; and an RS flip-flop, that controls the switching device. The power factor control circuit improves the power factor by keeping the average of the AC line current sinusoidal based on the full-wave-rectified AC input voltage and in the same phase with the AC input voltage.
When the switching device is ON, the full-wave-rectified AC line current flows to the ground via the switching device. The energy of the current is stored in the transformer. The multiplier determines a peak current value proportional to the AC input voltage and necessary for generating the average of the AC line current. The sensing current comparator compares the determined peak current value and the AC line current with each other. The sensing current comparator generates a reset signal when the AC line current increases to reach the peak current value determined by the multiplier. The reset signal resets the RS flip flop. The reset RS flip flop switches off the switching device. In response to the switching-off of the switching device, the current energy stored in the transformer is fed to the rectifying and smoothing circuit on the output side. The signal from the secondary side of the transformer, the level thereof is high while the switching device is ON, sets the RS flip flop. The set RS flip flop switches the switching device on. By setting and resetting the RS flip flop repeatedly, a current is fed to the rectifying and smoothing circuit on the output side.
Since the peak value of the AC line current is limited to a value proportional to the AC input voltage while the switching current is ON, the wave form of the average AC line current is a sinusoidal one similar to the sinusoidal wave of the AC input voltage.
It is necessary for the power supply circuit to have a starting circuit that facilitates start and restart of the power supply circuit. Usually, the conventional power supply circuit is provided with an exterior starting circuit, that employs an exterior oscillator. However, the exterior oscillator and such exterior parts and components increase the costs of the power supply circuit. Japanese Unexamined Laid Open Patent Application H06-86555 discloses a power supply circuit, that incorporates an interior starting circuit to obviate the above described problem.
The conventional power supply circuit uses a feedback signal obtained by dividing the DC output voltage with resistance. When the feedback signal is low, the power supply circuit boosts the DC output voltage. When the feedback signal is high, the power supply circuit lowers the DC output voltage. Since the power supply circuit works based on the principles as described above, the power supply circuit also boosts the DC output voltage when the feedback signal is short-circuited due to anomalies such as breakdown of the voltage divider for detecting the DC output voltage, causing dangerous states. To prevent such dangerous states, the conventional power supply circuit has an exterior comparator for monitoring the feedback signal. When the feedback signal is below the predetermined level, the output of the error amplifier inputted to the multiplier is set at zero. As a result, the peak value of the output from the multiplier becomes extremely small, resetting the RS flip flop and switching off the switching device.
Since the AC input voltage to the power supply circuit, that controls the power factor, is a full-wave-rectified sinusoidal wave, the AC input voltage is almost zero in the bottom portion of the sinusoidal wave. Therefore, the output voltage from the power supply circuit ought to be zero when the AC input voltage is in the bottom portion thereof. However, the conventional power supply circuit outputs a nonzero current which does not correspond to the zero input voltage, since the sensing current comparator outputs a low current due to the offset voltage outputted from the multiplier or the offset voltage inputted to the sensing current comparator. When the load of the power supply circuit is light, the peak value of the sinusoidal wave of the average current is low. Therefore, especially when the load of the power supply circuit is light, the nonzero current outputted corresponding to the bottom portion of the AC input voltage is troublesome. More specifically, the nonzero output current impairs the power factor.
The conventional power supply circuit, that incorporates a starting circuit, monitors the output of the RS flip flop storing the driving state of the exterior switching device. When the RS flip flop is in the reset state and it outputs for a predetermined period of time, the conventional power supply circuit sets the RS flip flop for restarting. However, a delay to the change of the output from the conventional power supply circuit occurs in the response of the timer circuit.
The conventional power supply circuit has an exterior detector for detecting a short-circuit of the feedback signal. When the exterior detector detects a short-circuit of the feedback signal, the exterior detector stops the conventional power supply circuit. These exterior parts and components increase the costs of the self-excited power supply circuit.
In view of the foregoing, it is an object of the invention to provide a power supply circuit that facilitates improving the power factor when the load of the power supply circuit is light.
It is another object of the invention to provide a power supply circuit that does not have any exterior parts and components for starting and restarting but incorporates a starting circuit that reacts quickly to the changes of the output from the power supply circuit.
It is still another object of the invention to provide a power supply circuit that facilitates reducing the parts and components for detecting the short-circuit of the feedback signal.
According to an embodiment of the invention, there is provided a power supply circuit, the power supply circuit switching on and off a full-wave-rectified AC input voltage to obtain a desired DC output voltage, the power supply circuit including: a power factor control circuit, the power factor control circuit keeping the average of an AC line current sinusoidal based on the full-wave-rectified AC input voltage and in the same phase with the AC input voltage.
Preferably, the power factor control circuit includes an offset regulating current generator, that generates an offset regulating current when the load of the power supply circuit is light; and a sensing current comparator, that compares a signal proportional to the AC input voltage and a signal indicating an AC line current to generate a reset signal, and cancels the offset voltage inputted thereto based on the offset regulating current inputted from the offset regulating current generator.
The power supply circuit having the structure described above, that includes an offset regulating current generator for generating an offset regulating current when the load of the power supply circuit is light and a sensing current comparator for canceling the offset voltage inputted thereto based on the offset regulating current inputted from the offset regulating current generator, facilitates correcting the output characteristics thereof when the load is light and improving the power factor.
Preferably, the power factor control circuit further includes a timer, that monitors a zero-cross signal and outputs a start signal or a restart signal when the zero-cross signal is not detected for a certain period of time, an RS flip flop, that outputs an ON-OFF signal for switching on and off the switching device of the power supply circuit, and an OR gate, that outputs a signal indicating the logical sum of the start signal or the restart signal from the timer and the ON-OFF signal from the RS flip flop.
The starting circuit for the power supply circuit, having the structure as described above, does not need any exterior parts and components, and it facilitates quick response to the changes of the output from the power supply circuit.
Preferably, the power factor control circuit further includes a short-circuit detection circuit, that monitors the feedback signal and outputs a short-circuit detection signal when the feedback signal is lower than a predetermined value, and an AND gate, that interrupts the ON-OFF signal for switching on and off the switching device of the power supply circuit based on the short-circuit detection signal inputted from the short-circuit detection circuit.
It is not necessary for the power supply circuit, having the structure as described above, to employ any exterior starting circuit.